直接上代码

+(void)addSRTWithSRTArr:(NSArray *)srtArr toVideoURL:(NSURL *)videoUrl complettion:(void (^)(NSURL *))completion

{
    NSURL *outPutUrl;
    
    
    //1 创建AVAsset实例 AVAsset包含了video的所有信息 self.videoUrl输入视频的路径
    AVAsset *videoAsset = [AVAsset assetWithURL:videoUrl];
    //2 创建AVMutableComposition实例. apple developer 里边的解释 【AVMutableComposition is a mutable subclass of AVComposition you use when you want to create a new composition from existing assets. You can add and remove tracks, and you can add, remove, and scale time ranges.】
    AVMutableComposition *mixComposition = [[AVMutableComposition alloc] init];
    
    //3 视频通道  工程文件中的轨道，有音频轨、视频轨等，里面可以插入各种对应的素材
    AVMutableCompositionTrack *videoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo
                                                                        preferredTrackID:kCMPersistentTrackID_Invalid];
    //把视频轨道数据加入到可变轨道中 这部分可以做视频裁剪TimeRange
    [videoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration)
                        ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeVideo] firstObject]
                         atTime:kCMTimeZero error:nil];
    
    //3.1 AVMutableVideoCompositionInstruction 视频轨道中的一个视频，可以缩放、旋转等
    AVMutableVideoCompositionInstruction *mainInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
    mainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, videoAsset.duration);
    
    // 3.2 AVMutableVideoCompositionLayerInstruction 一个视频轨道，包含了这个轨道上的所有视频素材
    AVMutableVideoCompositionLayerInstruction *videolayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoTrack];
    AVAssetTrack *videoAssetTrack = [[videoAsset tracksWithMediaType:AVMediaTypeVideo] firstObject];
    UIImageOrientation videoAssetOrientation_  = UIImageOrientationUp;
    BOOL isVideoAssetPortrait_  = NO;
    CGAffineTransform videoTransform = videoAssetTrack.preferredTransform;
    if (videoTransform.a == 0 && videoTransform.b == 1.0 && videoTransform.c == -1.0 && videoTransform.d == 0) {
        videoAssetOrientation_ = UIImageOrientationRight;
        isVideoAssetPortrait_ = YES;
    }
    if (videoTransform.a == 0 && videoTransform.b == -1.0 && videoTransform.c == 1.0 && videoTransform.d == 0) {
        videoAssetOrientation_ =  UIImageOrientationLeft;
        isVideoAssetPortrait_ = YES;
    }
    if (videoTransform.a == 1.0 && videoTransform.b == 0 && videoTransform.c == 0 && videoTransform.d == 1.0) {
        videoAssetOrientation_ =  UIImageOrientationUp;
    }
    if (videoTransform.a == -1.0 && videoTransform.b == 0 && videoTransform.c == 0 && videoTransform.d == -1.0) {
        videoAssetOrientation_ = UIImageOrientationDown;
    }
    [videolayerInstruction setTransform:videoAssetTrack.preferredTransform atTime:kCMTimeZero];
    [videolayerInstruction setOpacity:0.0 atTime:videoAsset.duration];
    
    // 3.3 - Add instructions
    mainInstruction.layerInstructions = [NSArray arrayWithObjects:videolayerInstruction,nil];
    //AVMutableVideoComposition：管理所有视频轨道，可以决定最终视频的尺寸，裁剪需要在这里进行
    AVMutableVideoComposition *mainCompositionInst = [AVMutableVideoComposition videoComposition];
    
    CGSize naturalSize;
    if(isVideoAssetPortrait_){
        naturalSize = CGSizeMake(videoAssetTrack.naturalSize.height, videoAssetTrack.naturalSize.width);
    } else {
        naturalSize = videoAssetTrack.naturalSize;
    }
    
    float renderWidth, renderHeight;
    renderWidth = naturalSize.width;
    renderHeight = naturalSize.height;
    mainCompositionInst.renderSize = CGSizeMake(renderWidth, renderHeight);
    mainCompositionInst.instructions = [NSArray arrayWithObject:mainInstruction];
    mainCompositionInst.frameDuration = CMTimeMake(1, 30);
    
    //加水印
    // videoLayer是视频layer,parentLayer是最主要的，videoLayer和字幕，贴纸的layer都要加在该layer上
    CALayer *parentLayer = [CALayer layer];
    CALayer *videoLayer = [CALayer layer];
    parentLayer.frame = CGRectMake(0, 0, renderWidth, renderHeight);
    videoLayer.frame = CGRectMake(0, 0, renderWidth, renderHeight);
    [parentLayer addSublayer:videoLayer];
    for (SrtModel *srtModel in srtArr) {
        UILabel *subtitleLabel = [[UILabel alloc]initWithFrame:srtModel.rect];
        subtitleLabel.text = srtModel.title;
        subtitleLabel.textColor = srtModel.textColor;
        subtitleLabel.font = [UIFont fontWithName:srtModel.fontName size:srtModel.fontSize];
        // 这个是透明度动画主要是使在插入的才显示，其它时候都是不显示的
        subtitleLabel.layer.opacity = 0;
        CABasicAnimation *opacityAnim = [CABasicAnimation animationWithKeyPath:@"opacity"];
        opacityAnim.fromValue = [NSNumber numberWithFloat:1];
        opacityAnim.toValue = [NSNumber numberWithFloat:1];
        opacityAnim.removedOnCompletion = NO;
        
        CABasicAnimation *rotationAnimation;
        switch (srtModel.animationType) {
            case 0:
                
                break;
            case 1:
                rotationAnimation = [SubTitleAnimation moveAnimationWithFromPosition:CGPointMake(subtitleLabel.layer.position.x + 100, subtitleLabel.layer.position.y) toPosition:subtitleLabel.layer.position];
                break;
            case 2:
                rotationAnimation = [SubTitleAnimation moveAnimationWithFromPosition:CGPointMake(subtitleLabel.layer.position.x, subtitleLabel.layer.position.y + 50) toPosition:subtitleLabel.layer.position];
                break;
            case 3:
                rotationAnimation = [SubTitleAnimation narrowIntoAnimation];
                break;
            case 4:
                rotationAnimation = [SubTitleAnimation fadeInAnimation];
                break;
            case 5:
                rotationAnimation = [SubTitleAnimation transformAnimation];
                break;
                
            default:
                break;
        }
        CAAnimationGroup *groupAnimation = [CAAnimationGroup animation];
        groupAnimation.animations = [NSArray arrayWithObjects:opacityAnim, rotationAnimation, nil];
        if (CMTimeGetSeconds(srtModel.insertTime.start) == 0) {
            groupAnimation.beginTime = 0.01;
        }else {
            groupAnimation.beginTime = CMTimeGetSeconds(srtModel.insertTime.start);
        }
        groupAnimation.duration = CMTimeGetSeconds(srtModel.insertTime.duration);
        
        [subtitleLabel.layer addAnimation:groupAnimation forKey:nil];
        [parentLayer addSublayer:subtitleLabel.layer];
    }
    
    mainCompositionInst.animationTool = [AVVideoCompositionCoreAnimationTool
                                 videoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer inLayer:parentLayer];
    
    
    
    
    
    // 4 - 输出路径
    
    NSString *documentsDirectory = [self mp4FilePath];
    NSString *myPathDocs =  [documentsDirectory stringByAppendingPathComponent:
                             [NSString stringWithFormat:@"v_%@.mp4",[NSString dateToIDstring:[NSDate date]]]];
    outPutUrl = [NSURL fileURLWithPath:myPathDocs];
    
    // 5 - 视频文件输出
    AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition
                                                                      presetName:AVAssetExportPresetHighestQuality];
    exporter.outputURL = outPutUrl;
    exporter.outputFileType = AVFileTypeMPEG4;
    exporter.shouldOptimizeForNetworkUse = YES;
    exporter.videoComposition = mainCompositionInst;
    [exporter exportAsynchronouslyWithCompletionHandler:^{
        dispatch_async(dispatch_get_main_queue(), ^{
            //这里是输出视频之后的操作，做你想做的
            NSLog(@"完事了-----%f",[self getRealVideoAllTimeWith:outPutUrl]);
            completion(outPutUrl);
            
        });
    }];


}

如需详细代码，请留言，有任何问题请指正，谢谢！